sorry, I unthinking, on Russian wrote. the thing is, I'm trying to build Debian-arm on the operating system . and I will also compile on the arm Board. because I didn't understand the cross-compile. assuming I don't have a pc to Gentoo to download source komadai emerge gentoo-sources then how else can I get the source?

You can grab the sources from a distfiles mirror. The distribution files include Gentoo-specific patches. If you are trying to compile Debian/Armbian you may as well use Debian. If you want to generate your own toolchain and/or any number of packages greater than a few you will find it easiest to use crossdev.

Crossdev will let you emerge packages on your desktop machine but compile them for your target processor. This is helpful if software you need to run does not have pregenerated packages for your distribution of choice. Alternatively, you can rent ARMv8 server time from Scaleway and compile natively which is essentially what Ubuntu, Debian, etc. do.